Carbon nanotubes (CNTs) have attracted considerable attention since their discovery due to their outstanding physical and chemical properties. Various synthesis methods have been developed for the production of CNTs, including electric arc discharge, laser vaporization, and catalytic chemical vapor deposition (CCVD). Many previous reports have shown the possibility of CNTs production on a large scale at a low cost.
Different types of reactor configurations including fixed-bed micro-reactors or small fluidized-bed reactors with short contact times are employed for the CVD synthesis of CNTs. “Fixed bed”, “floating catalyst”, and “fluidized bed” method are most common processes for the CCVD growth of CNTs.
The efficiency of CNTs production in the fixed-bed reactor is severely limited by inhomogeneous gas-solid mixing along the catalyst bed, with the dense packing of the catalyst particle preventing both CNTs production and dissipation of by-products. This situation is worsened by the growing nanotubes forming a mat that covers the catalyst bed. These problems can be solved by fluidizing the catalyst particles in a confined reactor by using a carbon source gas heated to the synthesis temperature.
Gas-solid fluidization is extremely difficult for ultrafine particles (including nanoparticles), since inter-particle forces are greater than the hydrocarbon. This is because ultrafine particles adhere to each other tightly in all directions by van der Waals or other forces, which seriously limit their fluidization. Fortunately, when particle size decreases to nano-scale, things can be different. For nanoparticles that are not zero-dimensional or that can coalesce into fractal subsets, that inter-particle force varies significantly with the packing structure and can be exploited for fluidization of ultrafines.
Recent research demonstrated that in the nano-agglomerated fluidized bed reactor (NAFBR), sufficient growing space, uniform temperature and concentration distribution and good mass and heat transfer lead to much higher yield, more uniform macroscopic properties and relatively perfect micro-structures of carbon nanotubes.
In the present invention, advantages of both CVD in a fluidized bed reactor and a floating catalyst are employed for continuous production of carbon nanotubes using methane as the carbon source. The Fe/MgO catalyst is prepared in-situ in a hot zone of the fluidized bed reactor and the CNTs on the catalyst are continuously produced. The influence of operating parameters including ferrocene sublimation rate and temperature on quality of the produced CNTs is reported.